Modern wind turbines are commonly used to supply electricity into the electrical grid. Wind turbines of this kind generally comprise a rotor with a rotor hub and a plurality of blades. The rotor is set into rotation under the influence of the wind on the blades. The rotation of the rotor shaft either directly drives the generator rotor (“directly driven”) or through the use of a gearbox.
Wind turbines are often grouped together in so-called wind farms. It has become increasingly difficult to find suitable locations for wind farms on land. On many occasions, there has been a lot of opposition against placement of wind turbines due mainly to the noise produced by the wind turbines and aesthetic effects of the placement of wind turbines. Additionally, for wind turbines to be able work efficiently, a windy and open area free from trees and buildings etc. is needed which is not always readily available.
For many reasons, it has become more popular to place wind turbines or wind parks in sea, either close to the coast (near-shore) or offshore (sometimes referred to as “far-offshore”): larger areas are available offshore, the wind may be more constant and of higher velocity on sea than on land, and wind shear is generally reduced. Additionally, with reduced noise constraints, wind turbines can rotate at higher speeds.
In near-shore applications, the wind turbines may be fixedly placed in generally shallow water (depths of approximately 25 meters or less) on foundations in the sea bottom. In far-offshore applications, the wind turbines may be designed to be floating platforms. Various configurations for floating wind turbines are known. They may e.g. be divided along the way in which they try to achieve static stability: using ballast weight, mooring lines or buoyancy.
The platforms that try to achieve static stability using ballast generally comprise an elongated tower structure comprising ballast weight under the water line and a buoyancy tank more or less at the water line. An example is shown in FIG. 1. Examples of platforms that try to achieve static stability using the tension of the mooring lines are shown e.g. in WO2009/087200 and EP1348867. Platforms that aim to achieve stability through the use of distributed buoyancy comprise a plurality of buoyancy tanks distributed around the tower at the water line. DE10219062 shows an example of a platform of this type. All floating platforms are provided with some form of mooring and anchoring means. Hybrid forms of the described floating wind turbine configurations may also be used.
Floating platforms can perform movements and rotations along three axes. With reference to FIG. 2, the x, y and z-axis of a local coordinate system can be defined. The x-axis is generally determined by the wind direction. The z-axis is determined by the longitudinal axis of the wind turbine. The y-axis is perpendicular to both the x and z-axes.
A linear motion along the x-axis is generally called “surge”, a linear motion along the y-axis is generally called “sway”, and a linear motion along the z-axis is generally called “heave”. Rotational motion around the x-axis are generally referred to a “roll”, rotational motion around the y-axis are generally referred to as “pitch” (not to be confused with pitching of a blade, which is a rotation of a wind turbine blade around is longitudinal axis), and rotational motion around the z-axis are generally referred to as “yaw”.
Floating platforms may perform complicated motion patterns under influence of e.g. wind gusts, turbulent wind, wind shear, asymmetry due to icing on the blades, waves, and tidal streams. An additional source of loads that may induce motions or oscillations in a floating wind turbine platform is the pitch control of the wind turbine blades. A common control strategy of a pitch system in a variable speed wind turbine is to maintain the blade in a predefined “below rated pitch position” at wind speeds equal to or below nominal wind speed. Said default pitch position may generally be close to a 0° pitch angle. Above the nominal speed, the blades are rotated to maintain the aerodynamic torque delivered by the rotor substantially constant. The exact cut-in wind speed, nominal wind speed and cut-out wind speed may depend on the location of the turbine, the turbine design etc.
FIG. 3 illustrates the variable speed strategy. Until nominal speed, the pitch angle is not varied and equal to or close to zero. Above nominal speed, the pitch angle is varied to maintain the aerodynamic torque constant. At the same time, due to the change in pitch angle (and angle of attack) of the blades, the thrust on the wind turbine (in x-direction, as defined in FIG. 2) is reduced above the nominal wind speed.
When the wind speed varies above the nominal wind speed, the pitch angle constantly needs to be adjusted. This adjustment leads to an ever changing thrust on the wind turbine. An oscillating thrust may cause fore-aft oscillations of the wind turbine. These oscillations may either be surging oscillations (displacement in x-direction) or a pitching motion (rotation around the y-axis). Similar effects also take place in wind turbines on land or near-shore wind turbines fixed on a foundation in the sea. The variation of the pitch angle and the variation of thrust that comes with it may lead to fore-aft oscillations in onshore wind turbines as well.
Under certain circumstances, resonance of the wind turbine in such a fore-aft oscillation may become a problem. The kind of fore-aft oscillations that can occur and the problems they can cause depend e.g. on the configuration of the turbine platform, the tension of the mooring lines, the configuration of buoyancy tanks etc.
One known measure to reduce fore-aft oscillation in wind turbines is to vary the pitch control such that resonance can be avoided. A disadvantage of this method is that the maximum potential power output of the wind turbine is not achieved, because the ideal pitch angles are not employed. Secondly, even though such an alternative pitch control has in some cases successfully been applied in wind turbines placed on land, they may not always be successful in offshore applications. This is due to the fact that the oscillations may generally be of lower frequency for which there is a particular danger of resonance.
There thus still exists a need to provide an effective method for reducing fore-aft oscillations in offshore wind turbines and an offshore wind turbine adapted for this purpose.
Apart from the fore-aft oscillations previously described, under the influence of atmospheric conditions and waves, sideways oscillations (e.g. a swaying oscillation or a rolling oscillation) or yawing oscillations may also occur in offshore wind turbines. There also exists a need for a method for reducing these kinds of oscillations in offshore wind turbine and an offshore wind turbine adapted for this purpose.
It is an object of the present invention to at least partially fulfil the before-mentioned needs.